The invention relates to a brake drum implemented as a casting.
Brake drums of this type are commonly formed in one-piece from gray cast iron, i.e. the cylindrical drum casing and the radially inwardly extending flange connected at one end are rigidly connected to one another.
During braking, the drum casing expands radially on account of heating by brake shoes bearing frictionally against the inner wall of the drum. In the region in which the drum casing is rigidly connected to the flange, however, this expansion is prevented by the flange, and therefore the drum casing expands conically toward the open end. In this process, the inner wall of the drum casing, i.e. the friction surface, loses its cylindrical configuration, as a result of which the braking action is reduced to an unacceptable extent.
In addition, the expansion which is prevented by the flange in the connection region leads to stresses in the friction surface of the drum casing, with consequent heat cracks, which may even lead to bursting of the drum casing.
The deformation of the drum casing, which is also referred to as bending, also gives rise to a non-uniform contact pattern and non-uniform wearing of the engaging brake shoes and of the drum casing.
If the brake shoes are changed, it is therefore necessary to machine the drum casing, during which a cylindrical friction surface is created again by turning out.
In order to eliminate these indeed very serious disadvantages, it is proposed in EP 0 490 021 B1, for example, to stiffen the drum casing by concomitantly casting in circumferential steel rings. These steel rings are prefabricated in the form of a cage. However, this concept acting in the sense of a brace has not been proved to be suitable to the desired extent.
Other design solutions for stabilizing the drum casing, i.e. for avoiding the bending, provide for the drum casing to be stiffened, for which purpose a circumferential rib is integrally formed on the outer side thereof at least in the free end region.
However, this measure leads firstly to an increase in the mass of the brake drum as a whole and secondly, by suppression of the thermal expansion, to additional thermal stresses in the drum casing, which incidentally similarly applies for the above-mentioned embedding of steel rings.
A brake drum of a different generic type is known from DE 29 27 148 A1, this brake drum not being implemented as a casting but instead consisting of steel. In this document, substantially obstruction-free radial expansion of the drum casing is achieved in that the latter is connected to the flange such that it can move freely in the radial direction.
However, the fastening of the flange to the drum casing is very complicated and is an obstacle to optimized production in terms of its manufacturing costs. Irrespective of this, a steel brake drum of this type has operational and functional disadvantages compared to a cast brake drum, and therefore this brake drum is not a recommendable alternative.
The invention is based on the object of further developing a brake drum of the above-described type such that its service life is increased and its operational reliability is improved with little design and manufacturing outlay.
This and other objects are achieved by a brake drum implemented by a casting, having a cylindrical drum casing and a connected flange, for fastening to a wheel hub. The drum casing and the flange are materially separated from one another and are connected to one another such that they can move radially in relation to one another by cast-in sliding elements.
The material separation of the flange and of the drum casing by means of the sliding elements has the effect that unobstructed, thermally induced radial expansion of the drum casing is possible even in the case of a cast brake drum.
Here, it should be emphasized in particular that it is extraordinarily simple to produce this brake drum.
The sliding elements, which consist of a material with a melting point which is higher than that of the cast material of the drum casing and of the flange, and of metallic material, preferably of stainless steel, or non-metallic material, for example of ceramic, are inserted into the casting mold, and the brake drum is then cast as a one-piece component.
According to a preferred embodiment of the invention, the drum casing and the flange each have a toothed ring, the teeth of which engage with one another. Here, the sliding elements are arranged between the radial side faces and also the contact faces of the teeth which face toward one another.
The cast blank initially completely encloses the sliding elements. During subsequent machining, material is removed from a rim of the flange, which protrudes outward beyond the drum casing, by turning until the sliding elements are exposed and the teeth of the two toothed rings lie in a circumferential plane. Two separate components, which are movable radially with respect to one another, are formed as a result.
Appropriate shaping of the sliding elements provides a positive connection between the flange and the drum casing which acts in the axial direction. For this purpose, radially extending beads are provided in the sliding elements, for example, the beads permit the radial movement of the drum casing with respect to the flange but preclude axial movement.
A further aspect of the invention provides for each sliding element to be formed with two plies, wherein one of the two plies which lie one against the other is held positively on the drum casing and the other ply is held positively on the flange. In this case, too, the configuration is selected such that the drum casing can move radially without obstruction, in which case the faces of the two plies which lie against one another then shift with respect to one another.
If the sliding elements are formed with one ply, it is expedient for there to be a coating on one side, with which a connection between the sliding element and the associated casting is prevented.
Each sliding element can have a U-shaped outline and surround a tooth of the toothed ring of the drum casing on three sides, the web which connects the—in this respect parallel—legs resting on the base of a tooth space of the toothed ring of the flange which has formed.
In order to provide the drum casing with the possibility to expand radially in any case, i.e. even in the event of possible warpage of the sliding elements during casting, the legs of the U-shaped sliding element are arranged at an angle with respect to one another in the radial direction, such that a tapered portion, or inversely a widened portion, is produced in the direction of the axis of rotation of the brake drum.
The arrangement of the sliding elements between the teeth of the toothed rings makes a high load-bearing capacity possible, since it is merely necessary for torque-dependent compressive forces to be absorbed, which of course permits small dimensioning, in particular in terms of the material thickness.
In principle, the problems described in relation to the prior art which arise as a result of thermal loading are solved, this including in particular the considerable reduction in susceptibility to cracking and the prevention of tangential skewed wearing of the brake pads and of the drum casing.
The use of stainless steel as sliding elements prevents corrosion in the region of separation between the drum casing and the flange, as a result of which a very high service life of the brake drum is obtained.
The operational behavior of the brake drum is positively influenced by a further aspect of the invention, according to which cooling openings are created by forming the teeth of the drum casing in the projection region with the teeth of the flange. These cooling openings ensure a very good dissipation of heat in the region of connection between the drum casing and the flange.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.